feat: Inserat erstellen — direct listing flow for units and standalone

- New /supply/new-listing page (Path A: pre-filled from portfolio unit,
  Path B: standalone blank form); creates EXTERNAL_MARKET + sourceType DIRECT
- "+ Inserat" button on available units in PropertyDetailView navigates
  with prefilled address/area/rent
- Nav entry "Neues Inserat" added to supply sidebar
- SourceProvenancePanel hidden for DIRECT source listings (no external source)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/match-detail/SourceProvenancePanel.tsx

@@ -13,6 +13,7 @@ interface Props {
 
 export function SourceProvenancePanel({ property }: Props) {
   if (!property) return null
+  if (property.sourceType === 'DIRECT' || property.sourceMeta?.sourceType === 'DIRECT') return null
 
   const freshness = property.dataQuality?.freshness
   const freshnessMeta = freshness ? (FRESHNESS_META[freshness] ?? null) : null